As shown in FIG. 1, normally for high efficiency DC/DC application, a Schottky rectifier is externally added in parallel with a MOSFET device to prevent a parasitic P/N body diode in the MOSFET from turning on in order to achieve higher speed and efficiency. The requirement for the clamping effect is that the forward voltage of the Schottky rectifier Vf is less than the parasitic body PN diode (˜0.7V). Besides the Schottky rectifier, a Gate-Source clamp diode with a breakdown voltage lower than the gate oxide rupture voltage of the MOSFET are provided for gate oxide ESD (electrostatic discharge) protection. Moreover, a Gate-Drain clamp diode with a breakdown voltage lower than that of the MOSFET are provided for Drain-Source avalanche protection. However, assembly of those separately structures into single package with extra interconnection wires results in higher manufacturing cost, and poor performance due to increase in inductance from the extra interconnection wires.
Another constraint is that, when fabricating the structure shown in FIG. 1, the conventional deep gate trenches are encountering a technical difficulty of high gate charge. The gate charge can be reduced simply by decreasing trench depth and P-body depth. However, the decreases may lead to increase of Rds, and lower breakdown voltage in termination if conventional field is used. As illustrated in FIG. 2, if the trench gate is not etched deep enough to a depth that the difference (Td−Pd) between trench depth Td and P-body depth Pd is less than 0.3 μm, Rds will be significantly increased. In FIG. 2, there are two curves, the upper one represents no As I/I at the bottom of the trench, and the lower one represents there is an additional n-dopant doped area surrounding bottom of trench and connecting to P-body region as shown in FIG. 3, which is our invention for resolving the Rds increasing issue due to shallow trench, and the difference between the two curves will be discussed below.
Accordingly, it would be desirable to provide more integrated trench MOSFET device with embedded Schottky rectifier, Gate-Drain and Gate-Source diodes on single chip for device shrinkage and performance improvement, and at the same time, having lower gate charge, lower Rds and higher breakdown voltage.